Electromagnetic energy, such as laser light, is used to perform various medical procedures including the vaporization of hyperplastic prostate tissues. One optical device that is used in surgical tools that perform such medical procedures is a side fire optical fiber device, also known as a lateral delivery device.
Lateral delivery optical fiber devices are typically used to redirect delivered electromagnetic radiation in an off-axis direction from the longitudinal axis of the delivery fiber, typically at an angle of 70-90 degrees off axis. Conventional side fire optical devices operate by reflecting the electromagnetic radiation off of a beveled optical surface that is machined and polished directly upon the transmitting optical fiber conduit, exploiting total reflection at or below the critical angle as described by Snell's Law. The conditions for total reflection are typically maintained by protecting the output bevel with a circumferential protective cap typically made of fused quartz or fused silica. The redirected output laser light is transmitted through a transmitting surface on the protective cap to the surgical site.
During surgery, the surface of the cap is subjected to cycles of rapid heating and cooling as well as sustained heating. Thermal cycling can induce stresses in the cap that are large enough to induce fracturing, particularly where the cap harbors residual stress from manufacturing, i.e. the external cap has not been annealed following melt processing as is the case in U.S. Pat. No. 5,537,499 (Brekke), U.S. Pat. No. 5,562,657 (Griffin), U.S. Pat. No. 7,463,801 (Brekke and Brucker), and U.S. Pat. No. 8,073,297 (Griffin). Both transient and sustained high temperatures at the transmitting surface of the cap accelerate the endothermic absorption of alkali metal ions within the quartz that form the cap, lowering viscosity sufficiently to permit rearrangement of the amorphous glass into high crystobalite; the cap undergoes devitrification.
These thermally mediated failure modes are more problematic when newer surgical lasers that produce significantly higher average powers are utilized, e.g. 120 W holmium laser energy (2140 nm), 180 W “Greenlight” laser energy (523 nm), 200 W diode laser energy (980 nm), and further magnified when the device is involved in tissue contact surgery. The stresses in the side fire optical devices often result in the cap cracking, shattering or perforation by sloughing off of highly localized and intense devitrification.
Lateral delivery optical fibers for surgery have been described and produced for decades. Early lateral delivery fibers were simple in construction FIG. 1: an optical fiber 30 polished at an off-axis angle 15 of between 35 and 40 degrees about which a closed end 5 transparent tube 10, akin to a tiny test tube is affixed (the tube is often called a “protective cap”, the surface through which the light exits being referred to the “transmissive surface”). Deficiencies with this simple design were quickly recognized and strategies designed to mitigate these deficiencies were implemented with varying degrees of success. The example in FIG. 1 illustrates an embodiment of an invention first described in Japanese Pat. No. 61-64242 and later in U.S. Pat. No. 4,740,047 (Abe, et al.) where the original cylindrical transmissive surface of the cap, and the cap surface 180 degrees opposing the transmissive surface 20, are modified to planar surfaces.
As illustrated in FIG. 2, some rays of light imparting the curved side of an optical fiber after reflecting from the bevel surface within the simplest lateral delivery fibers encounter angles at or near the critical angle for total reflection (rays C and D in FIG. 3A) as defined by Snell's Law such that a significant portion of the energy that encounters the reflective bevel tip does not exit the fiber in the desired direction, but undergoes complex reflections within the tip instead, eventually exiting in a variety of directions other than the desired direction, with roughly 20% of the errant energy leaking in roughly the opposite direction from the desired output. Referring to FIG. 3A and FIG. 4, reflected rays imparting the cylindrical cladding 70 to air interface near the outer edge of the fiber core 60, as represented by rays C and D, are completely reflected and encounter the cylindrical wall again at low angles, only to be reflected again until, at points 80 and 90, where the rays encounter the bevel face once more, they are refracted and transmitted Ct and Dt out of the fiber tip in the wrong direction. More central rays as represented by A and B, are refracted by the cylindrical fiber's glass to air interface and transmitted At and Bt in the desired direction. All rays also undergo Fresnel reflections Af, Bf, Cf and Df upon ultimately exiting the fiber tip.
These complex reflections are repeated where the diameter of the fiber (glass diameter, usually the cladding) closely matches the diameter of the protective cap bore (which is not the case in Abe FIG. 1, due to the use of a relatively thick buffer coating of silicone 35 in optical fiber of the era), at the air to cap glass interface. Additional contributions to scattering in directions other than the intended output results from Fresnel reflection at the fiber core to cladding interface 65 (typically minor due to closely matched refractive indices), the cladding to air interface, the air to protective cap interface and the cap to working environment interface (again, minor due to the much closer match of the refractive indices of glass and saline irrigation fluid versus those for glass and air).
Additional distortion of the output results from the non-orthogonal off axis angle of emission and the cylindrical lens effects of the curved surfaces in the output pathway. In total, roughly 28% FIG. 2 of the energy imparting the fiber bevel exits at angles that are surgically useless, and potentially harmful, while the energy that is emitted in the desired direction is highly distorted. Rather than a round spot that diverges symmetrically, the spot is typically reminiscent of a crab with a roughly oval center (body) with radiating streaks (legs) and divergence is highly asymmetric.
Abe, et al. describe a strategy to mitigate the problem of unwanted reflections within the lateral fiber structure by eliminating the curvature of the cap outer diameter and equipping the transmissive surface with an antireflective coating and the surface 180 degrees opposing the transmissive surface with reflective coatings. This strategy does considerably reduce the output that is 180 degrees opposite of the desired direction but does not correct the distortion and fragmentation of the output in the desired direction. The expensive optical coatings are also short lived, being easily damaged in the surgical environment.
U.S. Pat. No. 5,428,699 (Pon) describes a more elegant, yet partial solution to the problem of unwanted reflections within the side fire fiber output FIG. 3B. Recognizing that the closely matching curvature of the fiber core and fiber cladding was the source of most of the exit angles at or near the critical angle, Pon pointed out that equipping the fiber with a thicker cladding 71 would greatly reduce these unwanted, complex reflections by displacing the glass to air interface away from the core 61. An embodiment of the invention described in Pon (Laserscope's LDD-Stat and other branding) was highly successful in the marketplace over the bulk of the '699 patent lifetime, in spite of the high cost resulting from using very expensive 1.4 CCDR (Cladding to Core Diameter Ratio) fiber, because the invention reduced the unwanted scatter output and distortions of the output spot by almost 75%.
Roughly contemporaneous with Pon, two patents, Brekke '499 FIG. 5 and Griffin '657 FIG. 6, taught another strategy for reducing unwanted reflections in side firing fibers: fusion of the fiber's outer glass diameter to the protective cap's inner diameter. Eliminating the large differences in refractive indices in the output path essentially eliminated the unwanted critical angle reflections (referred to as “Snell reflections” hereafter), Fresnel reflections and cylindrical distortions of the output. Essentially no back reflections exist for the inventions described and the output profiles are essentially oval with the relatively sharp edges typical of standard, axial fiber output profiles. Both inventions describe embodiments that may be produced with far lower cost fiber optic materials than required by Pon (1.1 CCDR and 1.05 CCDR) but both inventions also suffer the same flaw. Fusion 110 of the bevel tipped 105 fiber 95 to the cap 100, either directly (Brekke, FIG. 5) or through a glass sleeve (Griffin, FIG. 6), results in high residual stresses “frozen” within the assembly that cannot be removed; the fused features of the output are contiguous with heat labile polymers on the transmitting fiber optic conduit such that the assemblies cannot be thermally annealed.
These stresses were problematic at the higher average power settings of lasers in use a decade ago, where repeatedly and rapidly heating and cooling the side firing fiber caps amplified preexisting stresses and/or flaws, often causing fractures at the junctions of fused and un-fused portions of the assemblies. Modern surgical laser powers can deliver twice the average power of the former installed base, making the control of Snell and Fresnel reflections even more important and rendering inviable the solutions taught in '499, '657 and even '699.
Prior art '297 FIG. 7 teaches a side fire optical device for laterally redirecting electromagnetic radiation-comprising: a cap member 150 comprising a closed end section 135, a tube section having a bore 115, and a transmitting surface 145; a sleeve 130 received within the bore of the tube section, the sleeve including a bore 133 and an exterior surface 137 that is fused to a surface of the bore of the cap 138 member; and a fiber optic segment 125 comprising an exterior surface 142 that is fused to a surface of the bore of the sleeve 143, a beveled end surface 140 positioned adjacent the transmitting surface of the cap member and a receiving end 144 opposite the beveled end surface that is within the bore of the tube section, wherein the beveled end surface is angled relative to a longitudinal axis of the fiber optic segment such that electromagnetic radiation propagating along the longitudinal axis of the fiber optic segment is reflected by the beveled end surface at an angle that is transverse to the longitudinal axis and through the transmitting surface of the cap member and variations thereof. Minor Fresnel reflections remain due to the lower refractive index of the fiber optic segment cladding 120 relative to the fiber optic segment core 125 and the sleeve 130 and at the fuses surfaces (due to contamination, captured gases, differential surface chemistry, etc.).
In particular, the fiber optic segment 125 of the prior art illustrated in FIG. 7 must be produced from a very limited selection of standard optical fiber materials unless custom drawn optical fiber is utilized. Standard optical fiber raw materials for constructing the fiber optic segment 125 are produced with buffer coatings and jacket materials. These polymers that must be removed completely, without damaging the exterior surface 142 of the fiber optic segment least gas bubbles form in the fusion process. Custom draws of optical fiber typically require a large minimum order and command premium prices; in the current marketplace, a minimum order custom optical fiber for the prior art '297 would provide sufficient material for approximately 250,000 to 1 million assemblies, sufficient devices for supplying 100% of the US market for 1 to 4 years.
Side fire fibers that are currently available to surgeons are exclusively single use devices that are discarded post-operatively and cost as much as $900 each. More than one fiber is often required to achieve the surgical goal, particularly in benign prostatic hyperplasia (BPH) cases where the patient has been taking drugs such as Flomax for relief of BPH symptoms, the prostate gland is larger than 30 grams and/or the patient has had a prior prostate resection. The fiber optic conduit and laser connector represent roughly 95% of the materials costs and between 20% and 40% of the labor costs of producing a side fire fiber.